Apparatus and method for fuel cell start from freezing without melting ice

ABSTRACT

Fuel cell systems ( 100, 400 ) and related methods involving accumulators ( 106, 200, 300, 406 ) with multiple regions (R 1 , R 2 ; R 1 ′, R 2 ′) of differing water fill rates are provided. At least one accumulator region with a relatively more-rapid fill rate (R 2 ; R 2 ′) than another accumulator region (R 1 ; R 1 ′) is drained of water at shutdown under freezing conditions to allow at least that region to be free of water and ice. That region is then available to receive water from and supply water to, a fuel cell ( 102; 402 ) nominally upon start-up. The region having the relatively more-rapid fill rate (R 2 ; R 2 ′) may typically be of relatively lesser volume, and may be positioned either relatively below or relatively above the other region(s).

BACKGROUND

The disclosure relates generally to fuel cells, and more particularly tofuel cell systems, accumulators therefor, and related methods. Moreparticularly still, the disclosure relates to such fuel cell systems andaccumulators for operation in sub-freezing temperatures.

Fuel cell systems, such as fuel cell power plants that provide power tothe propulsion system of electric vehicles, must be operable attemperatures below that at which water will freeze. Fuel cell systemsusing proton exchange membranes (PEM) are most typically utilized insuch applications, and require a well-hydrated membrane for goodoperation and durability. During normal operation, water may be drawnthrough a PEM fuel cell from the anode to the cathode. Notably, water isalso produced at the cathode. Traditional methods of operating fuelcells that are shut down in environments which may reach freezingtemperatures involve draining some or all of the water out of the fuelcell into a reservoir of some sort, as for example an accumulator.Before trying to establish subsequent operation, frozen water must bemelted before it can be moved back into the fuel cell, which process maytake at least several minutes, resulting in a delay that is likely to beviewed as intolerable for such transportation usage, where delayspreferably do not exceed a few seconds. Moreover, the melting of thefrozen water typically requires the application of a source of energythat may adversely impact the efficiency of the system.

One approach to deal with a facet of these concerns is described in PCTInternational Published Application WO 2006/112833 A1, wherein the cellsinclude porous water transport plates, and water contained only in thewater channels, ducts or pores of cells during operation is retainedthere by capillary action and/or by vacuum during shutdown, and servesto provide some humidification and cooling at start-up. While this mayat least partly address some problems, it relies upon the fuel cellshaving either coolant ducts of small size for the capillary action orthe use of a vacuum, or both, to prevent water from “slumping” into thereactant channels of the fuel cells. Additionally, it allows some amountof freezing of the water within the cells themselves during shutdown, solong as it is limited to the coolant ducts and not the reactantchannels. While this may be tolerable, it is less than desirable becauseit increases the time needed to bring the cell to operating temperature.Moreover, it does not really address the need to relatively rapidly andefficiently provide water from a reservoir, such as an accumulator, tothe fuel cells at, or shortly after, start-up, under freezingconditions.

SUMMARY

It has been determined that a fuel cell system with porous watertransport plates can be started, particularly though not exclusively,under freezing conditions, if a small amount of water can be madeavailable to the fuel cells from the accumulator relatively soon afterstarting. This is particularly true for evaporatively cooled systemsthat are less dependent on large volumes of water flow and can,therefore, be operated with less water stored in the system. In thatregard, the presently disclosed system(s) provide an accumulatorstructured to assure that at least a minimum usable quantity of water isavailable for supply to the fuel cells quickly, in an acceptableinterval, after start-up occurs, even under freezing conditions. This isaccomplished without resort to supplemental thawing means, even thoughsome frozen water may exist in part of the accumulator.

A fuel cell system subject to operation under freezing conditions isdisclosed in which there is at least one fuel cell, and an accumulatoroperative to receive water from and supply water to, the at least onefuel cell. The water from the fuel cell may go directly to the fuelcell, or may be delivered indirectly, as via a condenser or the like.The accumulator has structure defining a first region having a firstvolume configured to contain a volume of water during steady stateoperation of the at least one fuel cell and configured to fill withwater vertically at a first rate for an arbitrary fill water flow rate,and structure defining a second region having a second volume andconfigured to fill with water vertically at a second rate greater thansaid first rate for said same arbitrary fill water flow rate. Theaccumulator also includes a drain in the second region, said drain beingconfigured and operative to allow water to drain substantially entirelyfrom at least said second region following shutdown of the at least onefuel cell, thereby to prevent water from freezing in at least saidsecond region. The volume of the first region is typically, though notnecessarily, greater than the volume of the second region.

In one example embodiment, the accumulator second region is positionedlower than the accumulator first region and includes a geometry,relative to the geometry of the first region, that fills with watervertically more rapidly than does the first region at the same fillwater flow rate. This may be accomplished by the walls or sides of atleast the accumulator second region being arranged to define across-sectional area that is generally less than the cross sectionalarea defined by the walls or sides of the accumulator first region. Forexample, the accumulator first region may be defined by near-vertical orslightly inwardly inclined walls, and the walls of the accumulatorsecond region may extend downward there from in and range from nearvertical to inward inclination at a relatively greater angle than forthe first region. Alternatively, the walls of both the accumulator firstand second regions may have a common inwardly inclined contour, as forexample an inverted cone, with the accumulator second region being lowerthan the accumulator first region.

In another example embodiment, the accumulator second region may bepositioned or located somewhat in, and/or relatively above, theaccumulator first region, and includes a drain which allows water todrain, preferably passively, into the larger accumulator first region.This enables the accumulator second region to drain out all of its waterinto the accumulator first region upon shut-down, thus insuring that thesecond region is dry and does not freeze. The level of water accumulatedin the first region is regulated to remain below the second region toassure the ability of the first region to drain. The drain may be somesort of restriction, as an orifice or porous plug. Return lines fromboth of the accumulator first and second regions assure that one or bothregions may serve as sources of water for the fuel cells. The water inthe accumulator first region may be allowed to freeze upon shutdown,since the accumulator second region goes dry and is available to receivewater quickly upon start-up. This embodiment limits or eliminates theneed to dump water from the accumulator to prevent freeze-up.

In operation, upon shutdown of the fuel cells during freezingconditions, water is actively or passively drained from at least theaccumulator second region to leave it dry for rapid refilling with waterupon start-up (re-start) of the fuel cells. The active draining involvesthe opening and closing of a drain valve. Assuming the restart alsooccurs during cold conditions, fans associated with a condenser are setto high speed to maximize the water returned to the accumulator firstregion. The fuel cells can run sufficiently long under this coldcondition for water to be produced and collected in the accumulatorsecond region and then returned to the fuel cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting an exemplary embodiment of afuel cell system;

FIG. 2 is a schematic diagram depicting an exemplary embodiment of anaccumulator in accordance with FIG. 1;

FIG. 3 is a schematic diagram depicting a further exemplary embodimentof an accumulator in accordance with FIG. 1; and

FIG. 4 is a schematic diagram depicting a further exemplary embodimentof a fuel cell system, including accumulator.

DETAILED DESCRIPTION

Fuel cell systems and related methods involving accumulators withregions of relatively low volume and/or relatively high vertical fillrate are provided, several exemplary embodiments of which will bedescribed in detail. In this regard, in some embodiments, slopedsidewalls of the accumulators are used. As such, the sloped sidewallsencourage efficient draining of the accumulators. Additionally, the lowvolume and/or high fill rate regions require relatively less water tofill to a vertical height necessary for providing the water from theaccumulator. This potentially enables water filling the low volumeand/or high fill rate region to be directed for use in other portions ofa fuel cell system more quickly.

FIG. 1 is a schematic diagram depicting an exemplary embodiment of afuel cell system. As shown in FIG. 1, system 100 includes a fuel cellstack 102, sometimes called a cell stack assembly. Fuel cell stack 102includes multiple fuel cells, typically of the Proton Exchange Membrane(PEM) type. The fuel cell stack is positioned above a condenser 104,which is positioned over a water accumulator 106. An air inlet manifold108 is located above the fuel cell stack and an air outlet manifold 110is positioned below the stack. A fuel inlet manifold 120, a fuel turnmanifold 122 and a fuel outlet manifold 124 also are provided.

In operation, air enters air inlet manifold 108, flows through theoxidant flow channels of the fuel cell stack 102 to air outlet manifold110, and then into condenser 104. The outflow from condenser 104 isabove water line 112 of accumulator 106. Coolant for the condenser(illustrated by arrows 114) is ambient air in this embodiment. One ormore variable-speed fans 117 are associated with the condenser and arecontrolled by controller 119 to facilitate the rate at which moisture iscondensed from the air exiting the fuel cell stack. Cool dry air fromthe condenser is expelled from air outlet 116, which is located adjacentto a water overfill 118. In some embodiments, the condenser may functionas a manifold, in which case air outlet manifold 110 may be omitted.

During operation, fuel provided to fuel inlet manifold 120 flows to theleft, then through fuel turn manifold 122, after which the fuel flows tothe right. The fuel then flows out through fuel outlet manifold 124.Also, during operation, return water from the accumulator 106 flowsthrough a water conduit 126 to a water manifold 128. The water thenpasses into water channels within and/or adjacent to, the various fuelcells of the fuel cell stack 102 to the top of the fuel cell stack andpossibly to an upper water manifold 130. Notably, the embodiment of FIG.1 employs evaporative cooling. Thus, the only water entering through thewater manifold is to replace that which is evaporated into the airchannels of the fuel cell stack. A drain/drain valve 121 is located inthe bottom of the accumulator 106, or near thereto in the return waterconduit 126, and may be controlled by controller 119 to drain water fromthe system, as will be discussed further following.

The accumulator 106, in accordance with the present disclosure, includesat least structure defining a first region having a first volumeconfigured to contain a volume of water during steady state operation ofthe at least one fuel cell and configured to fill with water verticallyat a first rate for an arbitrary fill water flow rate, and structuredefining a second region having a second volume and configured to fillwith water vertically at a second rate greater than said first rate forsaid same arbitrary fill water flow rate. The volume of the accumulatorsecond region will typically be much smaller than that of theaccumulator first region, but need not be so if the water vertical fillrate relationship expressed above is otherwise met. The accumulatorfirst region will be hereinafter designated R1, or a derivative thereof,and the accumulator second region is designated R2, or a derivativethereof. It will be noted with reference to the embodiments of FIGS. 1-3that there may not be a clear demarcation of the transition from regionR1 to region R2, and thus they are depicted as overlapping. Indeed, withrespect to the detailed description of the accumulator embodiment ofFIG. 2, it is presented as having three regions, an “upper”, a “lower”,and an “intermediate”, but it should be understood that the intermediateregion could be viewed as all R1, all R2, or partly R1 and partly R2, solong as the guiding requirements expressed above are met.

FIG. 2 is a schematic diagram depicting an exemplary embodiment of anaccumulator similar to that depicted in FIG. 1. As shown in FIG. 2,accumulator 200 includes an upper region 202, an intermediate region 204and a lower region 206, which collectively comprise regions R1 and R2.Specifically, upper region 202 incorporates opposing endwalls 208, 210,and sloped sidewalls 212, 214 extending between the endwalls. Notably,lower portions (209, 211) of the endwalls define ends of theintermediate region. In the embodiment of FIG. 2, the sidewalls areinwardly inclined downwardly toward the intermediate region.

Intermediate region 204 is generally configured as a channel that runsalong the major central axis of the accumulator. The intermediate regionincludes opposing sidewalls 220, 222, and downwardly inclined bottomwalls 224, 226, which extend toward the lower region 206. Notably, lowerportions (221, 223) of the sidewalls 220, 222 function as sidewalls forthe lower region 206.

In this regard, the lower region 206 is defined by opposing sidewalls(lower portions of sidewalls 220, 222), endwalls 230, 232 that extendbetween the sidewalls, and a bottom, or drain, 234, for connection witha drain valve and return line, such as elements 121 and 126 of FIG. 1.The lower region defines a volume that is relatively small in comparisonto the overall volume of the accumulator. As noted previously, region R2may consist only of lower region 206, or it may additionally includepart of intermediate region 204.

FIG. 3 is a schematic diagram depicting an accumulator 300 that isfunctionally identical to that of FIG. 2, but which depicts the range oflatitude in selecting the structural configuration for the regions R1and R2. More specifically, whereas the accumulator 200 of FIG. 2 mayhave a relatively compact vertical profile, it requires a number ofplanar facets to construct, the inverted pyramidal accumulator 300 is ofdeeper vertical profile but relies on fewer and simpler members orfacets to construct. Indeed, the accumulator may be as simple in shape,though not necessarily simple to construct, as an inverted cone.Referring to FIG. 3, the accumulator 300 is formed principally of 4joined inverted triangular sides 310, 312, 314, and 316, and having asmall bottom, or drain, 320 at or near its apex, for connection with adrain valve and return line, such as elements 121 and 126 of FIG. 1.

FIG. 4 is a schematic diagram of a fuel cell system 400 generallyanalogous to FIG. 1, but in which the structure and function of theaccumulator 406 differ somewhat from that depicted and described withrespect to the FIGS. 1-3 embodiments. More particularly, the fuel cellstack 402, the condenser 404, and condenser fans 417 may be identical tothose discussed with respect to FIG. 1, and will not be describedfurther. On the other hand, the accumulator 406 positioned below theunderside of condenser 404 is structured to comprise a relatively largecontainer or chamber 410 having a bottom 412 for storage of the majorityof water provided by the condenser 404, and a further, typicallysmaller, container or chamber 414 supported or mounted within container410 and spaced above the bottom of container 410. The relatively largecontainer 410 may typically have a water level shown as 422 andestablished and controlled in a known manner which may include anoverflow vent 424 for establishing a maximum water level 422 and ventingany excess water. The smaller container 414 is positioned entirely abovethe level of that maximum water level established by the overflow vent424.

The smaller container 414 may be cup-like or bowl-like in shape andsupported from, or by, the relatively larger container 410, by inclinedsupport baffle 430 connected to each container. The support baffle 430is conveniently a substantially continuous annular surface forintercepting the majority of the water discharged from condenser 404 anddirecting it into the smaller container 414. Importantly, the generalcross-sectional area of the smaller container 414 is less, orconsiderably less, than that of the relatively larger container measuredat and below the typical water level 422, such that the former will havea relatively greater vertical fill rate than the latter for a given flowof water. To achieve this result in view of the large cross-sectionalarea of the support baffle, the smaller container 414 includes some formof overflow vent 432 positioned to limit the effective height of thatcontainer and thus assure the desired smaller cross-sectional area, andto additionally provide for rapid overflow of water to the largercontainer 410 if necessary. Notably, the smaller container 414 includesat its bottom a passive form of drain 440, which comprises a restrictionsuch as an orifice, a porous plug as shown, or the like.

As with the embodiments of FIGS. 1-3, the accumulator 406 of the FIG. 4embodiment includes a first region R1′ and a second region R2′ havingthe same functional relationships of vertical fill rates and possiblyalso volume as stated earlier, and it will be understood that it is therelatively larger container 410 that now constitutes the “first region”R1′ (below overflow vent 424) and the smaller container 414 constitutesthe “second region” R2′ (below overflow vent 432). This is so, despitecontainer 414 being positioned above portions of container 410.

A water conduit 450 extends from near the bottom of relatively largercontainer 410 to the water manifold 428 of the fuel cell stack 402 toreturn water to the fuel cell stack. A further water conduit 450Aextends from near the bottom of the smaller container 414 also to watermanifold 428, as by connection with conduit 450, to deliver/return waterto the fuel cell stack. A vacuum air pump 452 connected to the fuel cellstack 402 provides the delivery/return of water to the stack from eithercontainer 410 or 414 via the conduits 450, 450A, and may be controlledby a controller 419 which also controls the condenser fan(s) 417. Theseconduits 450, 450A also allow water to drain from the stack to theaccumulator 406 on shutdown. Optional valves 460 and/or 460A may beplaced in conduits 450 and/or 450A, respectively, to avoid excess gasingestion.

Very general reference is made first to aspects of operation common toall of the disclosed embodiments, and then separately to the distinctiveaspects of operation for the FIGS. 1-3 embodiments, and then further tothe distinctive aspects of the FIG. 4 embodiment. Referring to FIGS.1-4, at the time of shutdown of the fuel cell stack under freezing(including sub-freezing) conditions, there will typically be accumulatedcondensed water in accumulator regions R1 (or R1′) and R2 (or R2′). Thewater in at least region R2/R2′ is either actively or passively drainedto create an ice-free region of the accumulator, and is positioned andstructured to receive new water from the condenser relatively soon afterthe next start-up of the fuel cell stack. At start-up, the controller119, 419 causes the condenser fans 117, 417 to operate at high speed tomaximize water returned to the accumulator. Particularly in freezingconditions, the stack can operate safely at reduced temperature for theshort interval until sufficient water is produced. Even when ambienttemperatures are relatively warm, the stack can be started and operateeffectively, premised on relatively rapid refilling of the accumulatorwith water. At least region R2/R2′ is structured such that it canrelatively rapidly accumulate most or all of the new water, and it hasassociated with it a return conduit 126, 450A for conveying that waterback to the fuel cell stack to cool it.

Referring specifically to the FIGS. 1-3 embodiments, the drain valve 121connected to the bottom of region R2 of the accumulator is opened atshutdown, and serves to gravity-purge the entire accumulator (regions R1and R2) of water such that no ice is formed in the accumulator. Thevalve 121 is then closed. Upon subsequent start-up, water from the stack102 and condenser 104 is delivered to the accumulator 106, 200, 300whereupon it rapidly begins to fill region R2 for supplying the stackvia return water conduit 126. Because there was no water in theaccumulator, there is no ice that must be melted before start-up fromfreezing conditions.

Referring specifically to the FIG. 4 embodiment, most or substantiallyall of the water produced from condenser 404 falls to the downwardly andinwardly inclined support baffle 430 and is collected in the smallercontainer 414 comprising region R2′. That water rapidly fills thecontainer 414 and most simply overflows via the overflow vent 432 andfalls to the relatively larger container 410 where it accumulates asmain water level 422. Concurrently, a limited amount of the watercollected in smaller container 414 also slowly drains via the passivedrain 440 to the larger container 410 below. This slow drainage viadrain 440 is of little consequence during normal steady-state operation,but is important at both a frozen shut-down and upon a subsequent frozenstart-up. More specifically, upon a frozen shut-down, the watercollected in smaller container 414 slowly drains out of the container,to leave it dry and ice-free. While the water accumulated below in therelatively larger container 410 comprising region R1′ is allowed tofreeze if such conditions exist, this does not prevent a subsequenteffective start-up under those conditions. Rather, at start-up, newlycondensed water from stack 402 and condenser 404 soon fills theslow-draining smaller container 414, and water is thus available forreturn to the stack via water conduit 450A.

The optional valve 460, which may be located on the water return conduit450 from larger container 410 prior to its junction with the conduit450A from smaller container 414, serves when closed to prevent excessgas ingestion from container 410 while thawing is occurring there. Thatvalve should be located away from any ice that could form. Anotheroptional valve 460A can be located on the water return conduit 450A fromthe smaller container 414 to prevent, when closed, gas ingestion fromthat container, which might otherwise occur in that short interval justafter start-up when there is still relatively little or no water in thesmaller container 414. Control of each of these valves 460, 460A may beprovided by the controller 419. Similarly, control of the vacuum airpump 452 may be provided by the controller 419.

While certain aspects of the structure and function of the FIG. 4embodiment may be more complex than those of the FIGS. 1-3 embodiments,other aspects are less complex. Moreover, because the FIG. 4 embodimentoffers the ability to retain the water within the accumulator 406 duringa frozen shutdown, it minimizes or eliminates the need to dischargeliquid water overboard and may provide relatively more water morequickly in the event of a non-frozen/warm start-up, whether the priorshut-down was frozen or not.

Although the disclosure has been described and illustrated with respectto the exemplary embodiments thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions and additions may be made without departing from the spiritand scope of the disclosure.

1. A fuel cell system (100, 400) subject to operation under freezingconditions, comprising: at least one fuel cell (102, 402); and anaccumulator (106, 200, 300, 406) operative to receive water from andsupply water to, the at least one fuel cell, the accumulator havingstructure (202, 204; 310, 312, 314, 316; 410) defining a first region(R1, R1′) having a first volume configured to contain a volume of waterduring steady state operation of the at least one fuel cell andconfigured to fill with water vertically at a first rate for anarbitrary fill water flow rate, and structure (206, 204; 310, 312, 314,316; 414) defining a second region (R2, R2′) having a second volume andconfigured to fill with water vertically at a second rate greater thansaid first rate for said same arbitrary fill water flow rate, theaccumulator including a drain (121, 234, 320, 440) in the second region,said drain being configured and operative to allow water to drainsubstantially entirely from at least said second region followingshutdown of the at least one fuel cell, thereby to prevent water fromfreezing in at least said second region.
 2. The fuel cell system (100)of claim 1 wherein the accumulator second region (R2) is positionedbelow the accumulator first region (R1), the accumulator first region(R1) and the accumulator second region (R2) each having respectivevertical extents and having sidewalls that define respectivecross-sectional areas along the respective vertical extents, and whereinthe cross-sectional areas of the accumulator second region are less thanthose of the accumulator first region.
 3. The fuel cell system (100) ofclaim 2 wherein the sidewalls defining at least one of the accumulatorfirst region (R1) and the accumulator second region (R2) are inclineddownwardly inward.
 4. The fuel cell system (100; 400) of claim 1 whereinthe accumulator first region (R1) and the accumulator second region (R2)each having respective volumes, the volume of the accumulator firstregion being greater than the volume of the accumulator second region.5. The fuel cell system (100) of claim 1 wherein said drain in saidaccumulator second region (R2) includes a valve (121; 234; 320) that isoperative to permit selective drainage of the accumulator.
 6. The fuelcell system (100) of claim 5 wherein said valve of said drain isautomatically controlled by a controller (119).
 7. The fuel cell system(100; 400) of claim 1 wherein a water conduit (126; 450, 450A) isoperatively connected from at least the accumulator second region (R2)to the at least one fuel cell to return water to the at least one fuelcell.
 8. The fuel cell system (100; 400) of claim 1 further including acondenser (104; 404) and a variable speed condenser fan (117; 417), saidcondenser fan being operative at a maximum fan speed responsive to astart-up condition of the at least one fuel cell.
 9. The fuel cellsystem (100; 400) of claim 1 wherein the at least one fuel cellcomprises a stack (102; 402) of multiple Proton Exchange Membrane fuelcells.
 10. The fuel cell system (400) of claim 1 wherein the accumulatorsecond region (R2′) is positioned above the accumulator first region(R1′), the accumulator first region (R1′) and the accumulator secondregion (R2′) each having respective vertical extents and sidewalls thatdefine respective cross-sectional areas along the respective verticalextents, and wherein the cross-sectional areas of the accumulator secondregion are less than those of the accumulator first region.
 11. The fuelcell system (100) of claim 10 wherein water from the at least one fuelcell (402) is directed to the accumulator (406), and the accumulatorincludes structure (430) to direct most of said water from said at leastone fuel cell to said accumulator second region (R2) prior to saidaccumulator first region (R1′).
 12. The fuel cell system (100) of claim11 wherein said drain in said accumulator second region (R2′) comprisesa passive device (440) allowing a continuous restricted flow therethrough to said accumulator first region (R1′).
 13. The fuel cell system(100) of claim 12 wherein said passive device comprising said draincomprises a porous plug (440).
 14. The fuel cell system (100) of claim12 wherein the structure of said accumulator second region (R2′) furtherincludes an overflow arrangement configured to discharge water from theaccumulator second region to the accumulator first region (R1′) at arate greater than said continuous restricted flow via said passivedevice (440) when water in said accumulator second region exceeds apredetermined level.
 15. The fuel cell system (100) of claim 10 whereinwater conduits (450, 450A) extend from each of the accumulator firstregion (R1′) and the accumulator second region (R2′), and are eachoperative to return water to the at least one fuel cell.
 16. The fuelcell system (100) of claim 10 wherein the at least one fuel cellcomprises a stack (402) of multiple Proton Exchange Membrane fuel cells,and further including a condenser (404) and a variable speed condenserfan (417), said condenser fan being operative at a maximum fan speedresponsive to a start-up condition of the fuel cell stack (402).
 17. Afuel cell system (100, 400) subject to operation under freezingconditions, comprising: an accumulator (106; 200; 300; 406) operative toreceive water from a fuel cell (102, 402), the accumulator havingstructure (202, 204; 310, 312, 314, 316; 410) defining a first region(R1; R1′) having a first volume configured to contain a volume of waterduring steady state operation of the fuel cell and configured to fillwith water vertically at a first rate for an arbitrary fill water flowrate, and structure (206, 204; 310, 312, 314, 316; 414) defining asecond region (R2; R2′) having a second volume and configured to fillwith water vertically at a second rate greater than said first rate forsaid same arbitrary fill water flow rate, said first volume of saidfirst region (R1; R1′) being greater than said second volume of saidsecond region (R2; R2′), and the accumulator including a drain (121;234; 320; 440) in the second region, said drain being configured andoperative to allow water to drain substantially entirely from at leastsaid second region following shutdown of the at least one fuel cell,thereby to prevent water from freezing in at least said second region.18. A method of operating a fuel cell system at shutdown under freezingconditions, the fuel cell system having a fuel cell stack, and anaccumulator for receiving water from and supplying water to the fuelcell stack, comprising the step of: draining water from at least aregion of the of the accumulator at shutdown to allow the region to benominally water and ice-free during shutdown, whereby said region of theaccumulator may receive water from and supply water to the fuel cellstack nominally upon start up.
 19. The method of claim 18 including thestep of providing the accumulator with at least two regions, one of theat least two regions filling with water vertically more rapidly than another of the at least two regions for the same arbitrary fill flow rate,and wherein the step of draining at least a region of the accumulatorcomprises draining at least said one more-rapidly filling region. 20.The method of claim 19 wherein said one more-rapidly filling region ispositioned relatively above the other of the at least two regions, andthe step of draining the more-rapidly filling region is continuous via apassive drain discharging into at least the other of the at least tworegions.